I'm currently planning a book series that is told from the first person perspective of 5 different characters. I have a different voice for all of the characters, but I think that 2 of the characters should narrate in the present tense and the others in the past tense. Is that acceptable or is it too jarring for the reader? The only reason I want to do this is that these two characters are really fast-paced, in the moment kind of characters. Thanks for any input you have.
A writer can do nearly anything, but it must be clear to the reader what the writer is doing.
For instance, if you have a minor-character narrator in the past:
There are strange happenings today up at the Collins Mansion. Darkness has just fallen, and people are now scurrying about, lighting candles, and preparing for a party. Still, I can not be at ease with this, and am writing these words so that, should some evil befall me, some benefit may come from my observance.
For the "today" narrator:
I was going through granddad's desk and I found a thick, sealed envelope addressed to no one, but from someone named Cyrus. Granddad mentioned Cyrus to me just once, the Halloween we were talking about ghosts and demons. He said he'd always thought there was something odd about Cyrus. So, I opened the envelope and found many pages of writing in a very tight hand with what looks like fading iron gall ink. What I read makes sense. The old Collins place remains a dark cloud over this town, but unlike the weather no one discusses it.
This structure has made some time travel stories real. I don't know of some framework like this works for your story, but you have the power to make it fit without a crack or witness mark.
It would be inconsistent, but weird style/tone shifts have been done before, and not always to the detriment of the style. My example is the Edible Woman by Margaret Atwood, where the perspective goes from first person to third person, then back to first.
The reasoning is the main character starts off thinking she knows herself, then gets a rude awakening about her planned 'perfect' life and detaches from herself, up to and including having a dissatisfying affair, before finally finding herself again.
I think it's a mistake.
You certainly can do it and probably get away with it. But why make a tonal shift so severe if there's no real reason for it? Even though these are different characters, presumably interacting with each other, your reader will assume you're indicating different timelines with the tense switches (or the reader won't notice consciously but will just feel that something is "off").
You can write fast-paced action in either present or past tense. If this is your only reason for choosing to switch some of the narration, just practice writing past tense action. If it still doesn't work, consider switching the entire book to present tense (though really only do it if it works better overall, as present tense isn't used often for good reason).
It depends on the tense(s) of the five stories in relation to each other and the overall setting, I've seen narratives set in the past in the present tense when telling it in the first person after the fact, and it's quite effective. I've also seen characters tell past narratives in the past tense in a story otherwise told present tense as it unfolds.
So if you characters are relating events that happened earlier chronologically than the setting of the story, i.e. all your characters are sitting in a pub telling each other how they got there or similar, then you can mix and match the tenses they tell their individual stories in without it being too jarring as long as the speaker is clearly identified at any and all change over points between narratives.